Luminescent nanosheets, and fluorescent illuminators, solar cells and color displays utilizing the same as well as nanosheet paints

ABSTRACT

The invention provides a novel luminescent nanosheet, and applications to that luminescent nanosheet. The invention also breaks down conventional common knowledge of nanosheet solutions to provide a nanosheet paint that makes use of a high-concentration nanosheet solution suitable for dispersion of a luminescent nanosheet or the like. 
     The invention provides a luminescent nanosheet having perovskite octahedral crystals combined together in a planar configuration, characterized in that the octahedral crystals each have a multistacked crystal sheet structure wherein the octahedral crystals are multistacked over at least 3 high in the direction vertical to a sheet plane, and an element providing a luminescence center is solid-solubilized between the multistacked octahedral crystals (see FIG.  8 ). 
     The invention also provides a nanosheet paint having a nanosheet dispersed in a disperse medium, characterized in that an X value in the following equation 1 found from the vapor pressure of the disperse medium and the concentration of the nanosheet is in a range of less than 4.9×10 6  to greater than 3.8×10 3 : 
       X=C×V 4.01    1
 
     where C is the concentration (M) of the nanosheet, and V is the saturation vapor pressure (torr) of the solvent at 25° C. and 1 atm.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a luminescent nanosheet wherein ananosheet with perovskite octahedral crystals are combined together in aplanar configuration has a luminescence center element solid-solubilizedin it, and a fluorescent illuminator, a solar cell and a color displayutilizing that luminescent nanosheet as well as a nanosheet paintsuitable for dispersing a luminescent nanosheet or the like in adisperse medium.

2. Description of the Prior Art

Development of this type of luminescent nanosheet is now in mountingdemand because it allows an excitation source to be more likely to reachthe luminescence center element than a conventional particle typeluminescent substance.

As taught by Patent Publication 1, retaining the luminescence centerelement (ion) between nanosheets has been known (see FIG. 8(A));however, this has failed in making much use of energy from theexcitation source.

For this reason, solid-solubilization of luminescence center element insuch a nanosheet as shown in FIG. 8(B) is still desired.

The nanosheet shown in FIG. 8(B) has the luminescence center taken andconfined in a crystal structure; so it is more efficient than anarrangement with a luminescence center or the like sandwiched betweennanosheets in terms of transition of excitation energy from the host ornanosheet to the luminescence center. It could also have been confirmedthat luminescence features stabilized with respect to temperature andhumidity are achievable because of no need of intermediaries such aswater for transition of excitation energy from the host or nanosheet tothe luminescence center.

As shown in FIG. 2 of Non-Patent Publication 1, it has been well knownthat perovskite substances each take various rare earth ions in aperovskite structure as the luminescence center, turning into aluminescent substance. However, Non-Patent Publication 1 does not showsolid-solubilization of a rare earth luminescence center betweenoctahedral crystals of a multistacked crystal sheet structure whereinperovskite octahedral crystals are stacked over at least 3 high in thevertical direction to a sheet plane.

As set forth in Non-Patent Publications 2 and 3, it has been well knownthat alkaline metal ions in layered perovskite containing niobium ortantalum may easily ion exchange with other alkaline metal ions (Li⁺,Na⁺, Rb⁺, Cs⁺) or monovalent ions (NH₃ ⁺, Ag⁺, H⁺, n-C₈H₁₇NH₃, C₅H₅NH⁺,Tl⁺); however, neither of them shows solid-solubilization of the rareearth luminescence center between the perovskite octahedral crystals ofsuch a multistacked crystal sheet structure as described above.

Non-Patent Publications 4 and 5 have revealed that the rare earthluminescence center may be doped at not only a rare-earth site but alsoan alkaline metal site in the layered perovskite containing niobium ortantalum; however, neither of them show solid-solubilization of the rareearth luminescence center between the perovskite octahedral crystals ofsuch a multistacked crystal sheet structure as described above.

Referring to perovskite-type layers A₂Ta₃O₁₀ containing tantalum,Non-Patent Publications 6 and 7 have reported that although theelemental composition ratio of A to tantalum is basically 2, the amountof the element at the A site may be decreased (down to 16 mol %) orincreased (up to 22.5 mol %) by electrochemical reactions, acidtreatments or the like while the perovskite structure is kept intact;however, neither of them again show solid-solubilization of the rareearth luminescence center between the perovskite octahedral crystals ofsuch a multistacked crystal sheet structure as described above.

Non-Patent Publication 8 has unveiled synthesis of a triple perovskitenanosheet having a quadruple crystal sheet structure wherein perovskiteoctahedral crystals are stacked over 4 high in the vertical direction tothe sheet plane; however, it does not show solid-solubilization of theluminescence center between the octahedral crystals.

So far, nanosheets have been obtained by exfoliating a substance havingthe aforesaid layered structure and dispersing it in a disperse medium,as shown typically in Non-Patent Publications 9 and 10.

In other words, satisfactory dispersion of that substance in thedisperse medium has been considered as an essential requirement forobtaining a thinner nano-sheet; nanosheets have been created bydispersion using the disperse medium in an amount much larger than theamount of the nanosheet to be dispersed. In addition, such a nanosheetsolution has seemed to aggregate; it has been important for utilizationof that nanosheet to prevent the once thinned film from gainingthickness by reaggregation.

Therefore, when the obtained nanosheet solution that is of extremely lowconcentration is used as such, for instance when it is coated on a glasssubstrate or the like, it would be repelled, resulting in unsatisfactorycoating.

LISTING OF THE PRIOR ARTS Listing of the Patent Publications

-   Patent Publication 1: JP(A) 2004-285812

Listing of the Non-Patent Publications

-   Non-Patent Publication 1: Chemistry of Materials, Vol. 9, pp. 664,    1997, Kudo-   Non-Patent Publication 2: Materials Research Bulletin, Vol. 22, pp.    413, 1987, Gopalakrishnan et al.-   Non-Patent Publication 3: Solid State Ionics, Vol. 93, pp. 177,    1997, Toda et al.-   Non-Patent Publication 4: Materials Research Bulletin, Vol. 16, pp.    1429, 1981, Dion et al.-   Non-Patent Publication 5: Journal of Alloys and Compounds, Vol. 311,    pp. 159, 2000, Bizeto et al.-   Non-Patent Publication 6: Physica C, Vol. 455-448, pp. 26, 2006,    Kato et al.-   Non-Patent Publication 7: Journal of Physical Chemistry C, Vol. 112,    pp. 1312, 2008, Ozawa et al.-   Non-Patent Publication 8: Chemistry of Materials, Vol. 2, pp. 279,    1990, Treacy et al.-   Non-Patent Publication 9: Chemistry of Materials, Vol. 19, pp. 6575,    Ozawa et al.-   Non-Patent Publication 10: Journal of Physical Chemistry C, Vol.    112, pp. 1313, Ozawa et al.

SUMMARY OF THE INVENTION Objects of the Invention

An object of the invention, which such prior arts as described aboveunderlie, is to provide a novel luminescent nanosheet wherein aluminescence center element is solid-solubilized in a nanosheet havingperovskite octahedral crystals combined together in a planarconfiguration, and its applications. Another object of the invention isto break down such conventional common knowledge of nanosheet solutions,thereby providing a nanosheet paint using a high-concentration nanosheetsolution suitable for dispersion of luminescent nanosheets or the like.

Means for Accomplishing the Objects

The first aspect of the invention provides a luminescent nanosheethaving perovskite octahedral crystals combined together in a planarconfiguration, characterized in that said octahedral crystals each havea multistacked crystal sheet structure (shown in FIG. 8( c)) wherein theoctahedral crystals are multistacked over at least 3 high in thedirection vertical to a sheet plane, and an element providing aluminescence center is solid-solubilized between the multistackedoctahedral crystals.

According to the 2^(nd) aspect of the invention, the luminescentnanosheet of the 1^(st) aspect is further characterized in that saidperovskite octahedral crystals are each comprised of a tantalum oxide ora niobium oxide, and said luminescence center element is a rare earthelement (ion).

The 3^(rd) aspect of the invention provides a fluorescent illuminatorcomprising a fluorescent substance that receives excitation energy froman excitation source to emit out visible light having a givenwavelength, characterized in that said fluorescent substance is ananosheet as recited in Claim 1 or 2.

The 4^(th) aspect of the invention provides a solar cell that uses aphotoelectric device capable of generating electricity upon receipt oflight and has on a light-receiving surface side of said photoelectricdevice a photofilter that is excited by solar light to emit out lighthaving a wavelength different from that of solar light, characterized inthat said photofilter comprises a luminescent nanosheet as recited inClaim 1 or 2.

The 5^(th) aspect of the invention provides a color display comprising aluminescent substance that receives distinct excitation energies fromdistinct excitation sources to emit out light in distinct colors,characterized in that said luminescent substance comprises a luminescentnanosheet as recited in Claim 1 or 2, or a combination of it with otherluminescent nanosheet or other luminescent substance.

The 6^(th) aspect of the invention provides a nanosheet paint,characterized in that an X value in the following equation 1 found froma vapor pressure of a disperse medium and a concentration of thenanosheet is in a range of less than 4.9×10⁶ to greater than 3.8×10³:

X=C×V ^(4.01)   1

where C is the concentration (M) of the nanosheet, and V is thesaturation vapor pressure (torr) of the solvent at 25° C. and 1 atm.

Advantages of the Invention

The nanosheets of the 1^(st) and 2^(nd) aspects of the invention arefound to emit out light in quite distinct colors due to crystalstructure differences even with use of conventional luminescence centerelements (ions).

This could probably be ascribed to the facts that the efficiency oftransformation from the excitation source is much more improved thancould be achieved with a single type (shown in FIG. 8(B)) perovskitenanosheet, and that even factors that may change but have nothing to dowith emission colors are much more improved, giving influences toemission colors.

In any event, there is an unheard-of phenomenon hard to understand byreason of mere function improvements: the inventive substance should berecognized as a novel luminescent substance.

There is a novel finding underlying the inventive nanosheet paint thatthe nanosheet once dispersed in the disperse medium does not easilyaggregate; even when the concentration of the nanosheets is increased byremoval of a part of the disperse medium, it is quite unlikely that theyaggregate together, ridding the nanosheets of their own nature. It isthus possible to provide a solution containing the nanosheet in higherconcentrations.

In addition, those high concentrations mean that viscosity can also bekept high, eliminating repelling problems upon coating onto substrates.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a photograph taken of the Tyndall effect of a(K_(1.5)Eu_(0.5))Ta₃O₁₀ nanosheet that is diffusing laser beams.

FIG. 2 shows an in-plane diffraction pattern of a(K_(1.5)Eu_(0.5))Ta₃O₁₀ nanosheet, as found using radiation X-rays.

FIG. 3 shows (a) a TEM image and (b) a selected area electron-beamdiffraction pattern of a (K_(1.5)Eu_(0.5))Ta₃O₁₀ nanosheet,respectively.

FIG. 4 shows the shape of a (K_(1.5)Eu_(0.5))Ta₃O₁₀ nanosheet, asobserved under an atomic force microscope.

FIG. 5 shows an X-ray diffraction pattern of a condensed(K_(1.5)Eu_(0.5))Ta₃O₁₀ nanosheet, with a dotted line indicative of adiffraction pattern figured out on the basis of a tantalum doubleperovskite structure model.

FIG. 6 shows (a) an excitation spectrum (as measured by fluorescence at704 nm) and (b) a fluorescence spectrum (as excited at 314 nm) of a(K_(1.5)Eu_(0.5))Ta₃O₁₀ nanosheet, respectively.

FIG. 7 is a photograph taken of a (K_(1.5)Eu_(0.5))Ta₃O₁₀ nanosheetsuspension that is emitting out light by ultraviolet irradiation.

FIG. 8 is illustrative in schematic of relations of differences incrystal structure and luminescence center element positions: FIG. 8(A)shows a luminescence center element interposed between nanosheets, FIG.8(B) shows a luminescence center element solid-solubilized in a singleperovskite nanosheet as an example, FIG. 8(C) shows a luminescencecenter element solid-solubilized in a double perovskite nanosheet as anexample, and FIG. 8(D) shows a luminescence center elementsolid-solubilized in a double perovskite layered substance as anexample.

FIG. 9 is a graph indicative of differences in luminescencecharacteristics.

FIG. 10 shows (a) excitation spectra (as measured at 704 nm) and (b)fluorescence spectra (as excited by 322 nm ultraviolet radiation) ingray lines of a sample prepared by brush-coating a quartz glasssubstrate with a nanosheet solution having an increased concentration inExperiment No. 1. Note here that black lines are indicative of theexcitation and fluorescence characteristics of a luminescence nanosheetaqueous solution prior to being condensed by means of a centrifuge.

FIG. 11 is photographs of a sample prepared by writing a Chinesecharacter meaning “light” on a quartz glass substrate using theluminescent nanosheet of Experiment No. 1 and a brush, as taken under(a) white light and (b) ultraviolet radiation, respectively.

FIG. 12 shows an X-ray diffraction diagram for a glass sheet coated withthe luminescent nanosheet of Experiment No. 1 using a paintbrush.

FIG. 13 is a photograph illustrative of what state the sample ofExperiment No. 2 is coated on a quartz substrate in.

FIG. 14 is a photograph illustrative of what state the sample ofExperiment No. 3 is coated on a quartz substrate in.

FIG. 15 is a photograph illustrative of what state the sample ofExperiment No. 4 is coated on a quartz substrate in.

FIG. 16 is a graph indicative of the results of X-ray diffractions inExperiment Nos. 3 and 4.

FIG. 17 is a photograph illustrative of what state a nanosheet solutionin a 3×10⁻⁴ M ethanol solution in Example 4 is coated on a glasssubstrate in.

FIG. 18 is a photograph illustrative of what state a nanosheet solutionin a 6×10⁻² M ethanol solution in Example 4 is coated on a glasssubstrate in.

FIG. 19 is a graph indicative of the results of X-ray diffraction of ananosheet solution in a 6×10⁻² M ethanol solution in Example 4.

EMBODIMENTS FOR CARRYING OUT THE INVENTION

The present invention provides a luminescent nanosheet having perovskiteoctahedral crystals combined together in a planar configuration,characterized in that said octahedral crystals each have a multistackedcrystal sheet structure wherein the octahedral crystals are multistackedover at least 3 high in the direction vertical to a sheet plane, and anelement providing a luminescence center is solid-solubilized between themultistacked octahedral crystals. In Example 1 given later, Eu is usedas the element that provides a luminescence center; however, solidsolubilizing rare earth ions other than Eu in the crystal structure asthe luminescence center thereby enabling various emission colors to beobtained may be implemented by such well-known technology as set forthin Non-Patent Publication 1.

As known in the art, some perovskite crystals using niobic acid insteadof tantalum oxides in Example 1 provide similar luminescent substancestoo. According to this known technology, the starting material inExample 1 may be changed from tantalic acid to niobic acid therebyobtaining niobium oxide nanosheets of the double perovskite type (triplecrystal sheet structure). In other words, it is possible to emit outlight in a wide spread of colors by changing the types of rare earthions.

In Example 1 given later, K⁺ is used as the alkaline metal ions in thelayered perovskite. As set forth in Non-Patent Publications 2 and 3,however, K⁺ is susceptible of ion exchange with other alkaline metalions (Li⁺, Na⁺, Rb⁺, Cs⁺) or monovalent ions (NH₃ ⁺, Ag⁺, H⁺,n-C₈H₁₇NH₃, C₅H₅NH⁺, Tl⁺) so that there can be luminescent nanosheetsobtained that use as precursors a wide spread of double perovskiteniobium or tantalum oxide layered compounds making use of the aforesaidmonovalent cations.

Non-Patent Publication 4 has unveiled that not only the rare earth sitebut also the alkaline metal site in layered perovskite containingniobium or tantalum may be doped with rare earth luminescence centers.Given this known technology, it is possible to easily obtain not onlythe (K_(1.5)Eu_(0.5))Ta₃O₁₀ luminescent nanosheet of Example 1 givenlater but also double perovskite niobium or tantalum oxide luminescentnanosheets comprising A₂Ta₃O₁₀ where A is selected from alkaline metals(Li, Na, K, Rb, Cs), alkaline earth metals (Mg, Ca, Sr, Ba) and rareearths (Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) andused in varying ratios.

Although the elemental composition ratio of A to tantalum in thosesubstances is basically 2, it is understood that, as set forth inNon-Patent Publications 6 and 7, the amount of the element at the site Amay be decreased (down to 16 mol %) or increased (up to 22.5 mol %) byelectrochemical reactions, acid treatments or the like while theperovskite structure is kept intact.

The double perovskite luminescent nanosheet (K_(1.5)Eu_(0.5))Ta₃O₁₀) isexemplified in Example 1. It is here to be noted that Non-PatentPublication 8 has revealed the synthesis of a triple perovskitenanosheet having a quadruple crystal sheet structure wherein one morestack of octahedral crystals is added to the double perovskite in thedirection vertical to the sheet plane to obtain a four-stackedarrangement.

From Example 1 given later, it has been found that the luminescencecenter element can be incorporated in just only the single perovskiteknown so far in the art but the double perovskite as well. Given theunderlying principles, there would be no difficulty in application to atleast quadruple perovskite; that is, it would be possible to easilysolid-solubilize the element serving as the luminescence center betweenthe octahedral crystals in the triple perovskite nanosheet known in theart or even between the octahedral crystals in the perovskite nanosheetwherein one more stack of octahedral crystals is provided.

The emission wavelength of a luminescent material with the Euluminescence center taken in it depends on both the host structure andthe atoms of which it is composed; so even when the Eu luminescencecenter is taken in a double perovskite tantalum, oxide nanosheet hostwhere there are deficiencies at each of other atom sites or replacementsby other elements, there are far red light emissions (near 704 nm)obtainable.

If the oxide of tantalum or niobium having a large atomic mass is usedas the host, it will then be readily expectable that consumption bylattice vibration of excitation energy from the effect of that heavyelement is much reduced. This will in turn result in efficienttransformation from excitation energy to luminescent energy.

Referring here to a silicon-based solar cell, the maximum value of itslight absorption is in the vicinity of 700 to 900 nm. Accordingly,A₂Ta₃O₁₀ luminescent nanosheets (A is an alkaline metal, an alkalineearth metal or Eu) capable of transforming ultraviolet into far redcolor (704 nm) may be used on filters for making photoelectrictransformation efficient.

In short, the inventive luminescent nanosheet and its applications areunderstood to embrace Example 1 given just below as well as a widespread of modifications to which the ensuing findings are readilyapplicable.

EXAMPLE 1 Synthesis

A double perovskite tantalum oxide luminescent nanosheet with Euincluded as the luminescence center in the crystal structure issynthesized by three processes. First of all, a double perovskitetantalum oxide K (K_(1.5)Eu_(0.5))Ta₃O₁₀ that becomes the firstprecursor is obtained by mixing together powders of the raw materialsK₂CO₃, Eu₂O₃ and Ta₂O₅ at 5:1:3, and applying a solid-phase reaction tothe mixture in a platinum crucible at 1,225° C.

By allowing the first precursor K(K_(1.5)Eu_(0.5))Ta₃O₁₀ to react withabout 2M nitric acid at room temperature for 3 days, it is changed intothe second precursor that is an acidic solid in which the alkaline metalin the first precursor is ion exchanged with H. Finally, the secondprecursor and an aqueous solution of a bulky, alkaline molecule:tetrabutylammonium hydroxide (TBAOH) are reacted under agitation at roomtemperature for 1 week to exfoliate the layered oxide precursor layer bylayer.

Thus, (K_(1.5)Eu_(0.5))Ta₃O₁₀ that is the double perovskite tantalumoxide luminescent nanosheet with the Eu luminescence center included inthe crystal structure is obtained.

Although the synthesis of the (K_(1.5)Eu_(0.5))Ta₃O₁₀ nanosheet is hereexemplified, it is to be understood that other A₂Ta₃O₁₀ nanosheets (A isan alkaline metal (Li, Na, K, Rb, Cs), an alkaline earth metal or a rareearth with or without deficiencies at A, Ta and O) may also be obtainedby similar processes.

TABLE 1 Experiment No. 1 1^(st) Precursor CompositionK(K_(1.5)Eu_(0.5))Ta₃O₁₀ Raw Material K₂CO₃:Eu₂O₃:Ta₂O₅ Mixing Ratio byMass 5:1:3 Solid-Phase Reaction Temp. 1,225° C. 2^(nd) PrecursorReaction Solution Nitric Acid Solution (2M) Reaction Time 7 DaysComposition* H(K_(1.5)Eu_(0.5))Ta₃O₁₀ Exfoliation Solution TBAOH AqueousSolution (0.085M) Reaction Time 3 Days Nanosheet Composition(K_(1.5)Eu_(0.5))Ta₃O₁₀ Layer Structure Three Layers Composition*:Presumed from the nanosheet composition obtained by exfoliation.

Estimation

The elemental composition in the condensate of the(K_(1.5)Eu_(0.5))Ta₃O₁₀ nanosheet synthesized under the above conditionsand indicated by Experiment No. 1 was estimated by EPMA. It hasconsequently been confirmed that the elemental composition ratio is1.5:0.5:3 for K:Eu:Ta, indicating that this nanosheet have thecomposition: (K_(1.5)Eu_(0.5))Ta₃O₁₀.

It has also been confirmed from the Tyndall effect that the synthesizednanosheet is colloidally diffused in the solution (FIG. 1). From theresults of in-plane X-ray diffraction using synchrotron radiationX-rays, it has further been confirmed that this nanosheet keeps intactthe perovskite-based structure of the bulk precursor (FIG. 2). And fromshape observation under a transmission electron microscope, the ensuingnanosheet has been found to have uniform thickness, and from the resultsof selected area electron diffraction, it has been found to keep intactthe perovskite-based structure of the precursor (FIG. 3). From shapeobservation under an atomic force microscope, this nanosheet has beenfound to have a uniform thickness of 2.4 (2) nm (FIG. 4). The X-raydiffraction of the condensate by a centrifuge of the nanosheetsuspension has matched well with that calculated on the basis of a(K_(1.5)Eu_(0.5))Ta₃O₁₀ tantalum double perovskite structure model,indicating that this nanosheet has a tantalum double perovskitestructure (FIG. 5).

FIG. 6 shows the fluorescence characteristics of the(K_(1.5)Eu_(0.5))Ta₃O₁₀ nanosheet having the strongest emissionintensity near 704 nm (far red). In many other luminescent substancesincluding the Eu luminescence center, red emissions near 612 nm due totransition from ⁵D₀ to ⁷F₂ have the strongest intensity, whereas in the(K_(1.5)Eu_(0.5))Ta₃O₁₀ nanosheet, far red emissions due to transitionon a high wavelength side from ⁵D₀ to ⁷F₄ of Eu³⁺ have the strongestintensity. From the fact that the emission wavelength at which thestrongest emission intensity is obtained depends on the structure of thehost with the luminescence center taken in it and the type of atoms ofwhich it is composed, far red emissions out of the(K_(1.5)Eu_(0.5))Ta₃O₁₀ nanosheet would be obtained through theincorporation of the Eu luminescence center in the tantalum doubleperovskite nanosheet host. From the excitation spectra of the(K_(1.5)Eu_(0.5))Ta₃O₁₀ nanosheet, it has been confirmed that emissionsby excitation of the nanosheet host near 314 nm are much higher inefficiency than by direct excitation of the Eu³⁺ luminescence center. Inaddition, this nanosheet is found to emit out light having a luminousintensity high enough to be visible by the naked eye (FIG. 7).

Differences Between the Single and the Double

Existing niobium or tantalum perovskite oxide nanosheets each have the(single perovskite) structure wherein octahedrons, each made up ofniobium or tantalum and oxygen, are stacked over 2 high in the directionvertical to the sheet (see FIG. 8(B)).

What is here provided has the (double perovskite) structure whereinoctahedrons are stacked over three high in the direction vertical to thesheet (see FIG. 8(C)). Now that the properties of a substance dependheavily on its structure, the inventive double perovskite oxidenanosheet (double perovskite oxide nanosheet doped with the Eu³⁺luminescence center: (K_(1.5)Eu_(0.5))Ta₃O₁₀) could possibly havephysical properties different from those of existing single perovskitenanosheets (single perovskite oxide nanosheets doped with the Eu³⁺luminescence center: Eu_(0.56)Ta₂O₇).

Actually with the same excitation source (314 nm wavelength light) used,the single perovskite nanosheet doped with the Eu³⁺ luminescence centergives out red emissions near 615 nm (FIG. 9( a)), whereas theexemplified double single perovskite nanosheet doped with the Eu³⁺luminescence center gives out far red emissions near 704 nm (FIG. 9(b)).

Referring then to the inventive nanosheet paint, a part of the dispersemedium is removed to make its concentration so high that it may also beapplied to increase the concentration of a nanosheet solution obtainedby exfoliation and dispersion of a substance having a layered structure.

Removal by centrifugal separation, evaporation of the disperse medium,etc. may be applied to this end.

It has here been revealed that coatability is governed by the X value inthe following equation 1 determined primarily by the concentration ofthe nanosheet and the vapor pressure of the disperse medium.

X=C×V ^(4.01)   1

where C is the concentration (M) of the nanosheet, and V is thesaturation vapor pressure (torr) of the solvent at 25° C. and 1 atm.

From Examples 2 to 4 given later, the upper limit to the X value inEquation 1 should be set at less than 4.9×10⁶, preferably 4.5×10⁶ orless, more preferably 4×10⁶ or less, and even more preferably less than3.3×10⁶ (M torr^(4.01)).

The lower limit, on the other hand, should be set at greater than3.8×10³, preferably 4×10³ or greater, more preferably 4.5×10³ orgreater, and even more preferably 5×10³ or greater (M torr^(4.01)).

It is here to be noted that the lower limit is less likely to beaffected by the vapor pressure of the disperse medium than the upperlimit; it may be determined by concentration alone, and in that case,the concentration of the nanosheet should be set at 1×10⁻² M or greater.

Being short of that lower limit will give rise to a demerit of the paintbeing repelled upon coating onto a substrate.

Concentrations higher than the upper limit to the X value will renderuniform thickness coating difficult.

As already well known in the art, a concentration/diffusion cycle inwhich after concentration, the nanosheet is diffused in other solvents(alcohol, acetone, hexane, etc.) is repeated thereby turning thenanosheet into nanosheet solutions (paints) in a variety of solvents;that is, it will be readily appreciated from Examples 2 to 4 given laterthat there can be a variety of dispersion media used.

The aforesaid Equation 1 works effectively, especially for a volatiledisperse medium whose vapor pressure is higher than that of water; forinstance when there is a disperse medium of high volatility such asethanol used, the solvent condenses at low densities because of rapidevaporation speed of the solvent during regulation of its concentrationby evaporation, resulting in the inability to reduce the volume in theconcentration=number of moles/volume down to a sufficient level. In thatcase, the paint may be regulated beforehand in terms of coatability,with this in consideration.

The present invention may also be applied to paints used with spincoating, screen printing, ink jet printing or other methods. With waterused as the disperse medium, the concentration of about 0.27 M (adensity of 1.3 g/cm³) is best.

Most preferable for Equation 1, X=8.9×10⁴ M torr^(4.01), and the mostpreferable range would be within±single-figure number from it.

In the examples given later, the K_(1.5)Eu_(0.5)Ta₃O₁₀ nanosheetsolution will be exemplified; however, it will be readily appreciatedthat conventional known nanosheets made up of other compositions, too,may bring about such inventive advantages as described above.

The present invention may be applied not only to nanosheets having afunction of emitting light but also to photocatalysts (for decompositionof organic matters, ultra-hydrophilic nature, decomposition of waterinto hydrogen and oxygen, etc.), high dielectric devices,room-temperature ferromagnetic devices, macro-magnetic opticalpolarization devices capable of responding to ultraviolet light, andmultilayered structure devices having orientation crystals formed onnanosheets as seed crystals, etc.

Especially in the aforesaid multilayered structure, the nanosheet isgenerally very large in terms of the ratio between its longitudinaldirection size (of micro-order) and its thickness (of nano-order) sothat a film with oriented crystals could likely be formed by simplecoating of the nanosheet paint, resulting readily in creation of asingle- or multi-layered structure having uniform orientation.

EXAMPLE 2

Example 2 is now explained with reference to a K_(1.5)Eu_(0.5)Ta₃O₁₀nanosheet solution as an example.

Synthesis

The nanosheet solution is obtained by soft chemical exfoliation of therespective layers of a layered oxide into individually independentlayers, as set forth in Publications 9 and 10. In this state, thenanosheet was found to have a concentration of 5.2×10⁻⁴ M.

This luminescent nanosheet aqueous solution was condensed by acentrifuge at the rpm and centrifuging time set out in Table 2, givenjust below. Then, the supernatant liquid was removed off to obtain ahigh-concentration nanosheet solution with the concentration and densityset out in Table 2.

TABLE 2 Results Experiment RPM, ×10³ rpm Centrifuging Density, of No.(G) Time, hours Concentration, M g/cm³ Coating 1    20 ½ 0.27 1.3Coating (36,000) on the quartz glass substrate

Estimation

The nanosheet solution (Experiment No. 1) synthesized under the aboveconditions could be brush-coated on a quartz substrate without beingrepelled.

It has been confirmed that the fluorescent feature of the sample beforecondensed by centrifugal separation into the paint is kept intact (FIG.10). It has just only been confirmed that the K_(1.5)Eu_(0.5)Ta₃O₁₀nanosheet paint gives out far red emission by ultraviolet irradiation,but it has also been confirmed from its excitation spectra that emissionby excitation of the nanosheet host near 322 nm is much higher inluminescent efficiency than that by direct excitation of the Eu³⁺luminescence center. Upon coated and dried on the substrate, thisnanosheet luminescent material is transparent, yet it gives out anemission intensity high enough to be visible by the naked eye underultraviolet light (FIG. 11). It has been confirmed from the results ofX-ray diffraction that in the nanosheet paint coated on the glasssubstrate, the nanosheet crystals are impeccably oriented such that theglass sheet plane is parallel with the nanosheet plane (FIG. 12). It hastherefore been verified that use of the nanosheet paint can result inready formation of a film with crystals oriented in place.

EXAMPLE 3

Example 4 is now explained with reference to an Eu_(0.56)Ta₂O₇ nanosheetsolution as an example.

Synthesis

A layered tantalum oxide of the perovskite type: Li₂Eu_(2/3)Ta₂O₇ thatbecomes the first precursor is obtained by mixing together powders ofthe raw materials Li₂CO₃, Eu₂O₃ and Ta₂O₅ at 2:2/3:3, and applying asolid-phase reaction to the mixture in a platinum crucible at 1,600° C.(an incremental 50° C.) in air.

By allowing the first precursor to react with about 2M nitric acid atroom temperature for 3 days, it is changed into the second precursorH₂Eu_(0.56)Ta₂O₇ that is an acidic solid in which Li⁺ in the firstprecursor is ion exchanged with H⁺, and some part of Eu was extracted.

Finally, the second precursor and an aqueous solution of a bulky,alkaline molecule: tetrabutylammonium hydroxide (TBAOH) are reactedunder agitation at room temperature for 1 week to exfoliate the layeredoxide precursor layer by layer, thereby obtaining Eu_(0.56)Ta₂O₇ that isa tantalum oxide luminescent nanosheet with the rare earth luminescencecenter included in the crystal structure. This luminescent nanosheetaqueous solution is condensed by a centrifuge at the rpm andcentrifuging time set out in Table 3, given just below, asnon-exfoliated matter or a supernatant liquid is removed off, wherebynanosheet solutions are obtained with the concentrations set out inTable 3.

TABLE 3 Experiment No. 2 3 4 Centrifuging ×10³ rpm 2.5 20 NA G 6 × 10²36 × 10² — Time (min.) 5 30 NA Contents of the Nanosheet Solution 2′  3′ 4′ Concentrations (M) 1 × 10⁻³  3 × 10⁻¹ 15 (A) Results Repelling (B)(C) Coating State References FIG. 13 FIG. 14 FIG. 15 Results NA (D) (E)Orientation References FIG. 16 FIG. 16 2′: Unexfoliated matter-freesupernatant liquid of the luminescent nanosheet aqueous solutionsubjected to centrifugal separation. 3′: Supernatant liquid-freecondensed nanosheet solution obtained by centrifugal separation of theExperiment No. 2 nanosheet solution. 4′: Pasty nanosheet obtained byevaporation of the solvent from the Experiment No. 3 nanosheet solution(poor in orientation capability). (A) Results of coating onto the quartzglass substrate. (B) Capable of being coated onto the quartz substrate.(C) Incapable of being coated due to pastiness. (D) Orientation parallelwith the substrate plane. (E) Poor in orientation capability.

Estimation

In an attempt of coating a substrate with the nano-sheet solutionssynthesized under the above conditions (Experiment Nos. 2 to 4) at aconcentration of 1×10⁻³ M, they are repelled (FIG. 13); however, thenanosheet paint condensed up to 3×10⁻¹ M can be coated on the substratewithout being repelled (FIG. 14). In the coated 3×10⁻¹ M nanosheetpaint, the nanosheet crystals are impeccably oriented such that thesubstrate plane is parallel with the nanosheet plane, as can be seenfrom the results of X-ray diffraction (FIG. 16). However, the paintcondensed up to 15 M by evaporation of the solvent (water here) justonly turns into a sticky paste having difficulty in uniform coating(FIG. 15), but it is also poor in crystalline and orientation features,as can be seen from the results of X-ray diffraction (FIG. 16).

EXAMPLE 4

Example 4 is here explained with reference to an ethanol dispersion of(K_(1.5)Eu_(0.5))Ta₃O₁₀ nanosheet as an example.

Synthesis

A double perovskite tantalum oxide K(K_(1.5)Eu_(0.5))Ta₃O₁₀ that becomesthe first precursor is obtained by mixing together powders of the rawmaterials K₂CO₃, Eu₂O₃ and Ta₂O₅ at 5:1:3, and applying a solid-phasereaction to the mixture in a platinum crucible at 1,225° C.

By allowing the first precursor K(K_(1.5)Eu_(0.5))Ta₃O₁₀ to react withabout 2M nitric acid at room temperature for 3 days, it is changed intothe second precursor that is an acidic solid in which the alkaline metalin the first precursor is ion exchanged with H. Finally, the secondprecursor and an aqueous solution of a bulky, alkaline molecule:tetrabutylammonium hydroxide (TBAOH) are reacted under agitation at roomtemperature for 1 week to exfoliate the layered oxide precursor layer bylayer.

An aqueous dispersion of the obtained double perovskite tantalum oxideluminescent nanosheet: (K_(1.5)Eu_(0.5))Ta₃O₁₀ may be centrifuged at2,000 rpm for 15 minutes, thereby removing non-exfoliated matter bysedimentation. The supernatant (K_(1.5)Eu_(0.5))Ta₃O₁₀ nanosheet aqueousdispersion free of non-exfoliated matter may be centrifuged at 20,000rpm for 30 minutes for removal of the supernatant liquid whereby a pastycondensate of (K_(1.5)Eu_(0.5))Ta₃O₁₀ nanosheet is obtained. Thirty (30)mL of ethanol are added to this condensate for re-dispersion, and the(K_(1.5)Eu_(0.5))Ta₃O₁₀ ethanol dispersion is then centrifuged at 20,000rpm for 30 minutes for removal of the supernatant liquid. The cycleinvolving re-dispersion into ethanol and re-condensation of thenano-sheet is once more repeated for making sure removal of the watercomponent. Finally, five (5) mL of ethanol are added to the condensed(K_(1.5)Eu_(0.5))Ta₃O₁₀ nanosheet to obtain a (K_(1.5)Eu_(0.5))Ta₃O₁₀nanosheet ethanol dispersion at a concentration of 6×10⁻² M (ExperimentNo. 5). Further, this 6×10⁻² M (K_(1.5)Eu_(0.5))Ta₃O₁₀ nanosheet ethanoldispersion is diluted 200 times with fresh ethanol to obtain a(K_(1.5)Eu_(0.5))Ta₃O₁₀ nanosheet ethanol dispersion at a concentrationof 3×10⁻⁴ M (Experiment No. 6).

Estimation

In an attempt of coating a glass substrate with a nanosheet solutioncomprising the 3×10⁻⁴ M (K_(1.5)Eu_(0.5))Ta₃O₁₀ ethanol dispersionprepared under the above conditions, it is repelled (FIG. 17); however,a nanosheet solution comprising 6×10⁻² M (K_(1.5)Eu_(0.5))Ta₃O₁₀nanosheet ethanol dispersion can be coated on, without being repelledby, a glass substrate (FIG. 18). In the coated 6×10⁻² M nanosheet paint,it has been confirmed that the nanosheet crystals are impeccablyoriented such that the substrate plane is parallel with the sheet plane,as can be seen from the results of X-ray diffraction (FIG. 19). As thesolvent ethanol is evaporated off to bring the nanosheet ethanoldispersion up to a concentration of 0.3 M or greater, it gets rid offluidity, resulting in the inability to coat.

Table 4 shows the results of figuring out the X value in Equation 1based on Examples 2, 3 and 4.

TABLE 4 Experiment No. 1 2 3 4 5 6 Nanosheet Concentration (M)  0.27  1× 10⁻³   3 × 10⁻¹ 15     6 × 10⁻²   3 × 10⁻⁴ Disperse Medium MediumWater Water Water Water Ethanol Ethanol Vapor Pressure (Torr) 23.7623.76 23.76 23.76 59.00 59.00 X Value 8.9 × 10⁴ 3.3 × 10² 9.9 × 10⁴ 4.9× 10⁶ 7.6 × 10⁵ 3.8 × 10³ Results of Coating ◯ Repelling ◯ Pasty ◯Repelling

APPLICABILITY TO THE INDUSTRY

The inventive luminescent nanosheet, because of being capable oftransforming excitation energy to luminescent energy with highefficiency, may find applications to fluorescent illuminators, solarcells, color displays or the like.

If distinct luminescent nanosheet paints according to the invention aremixed at varying ratios as is the case with coloring materials, it isthen possible to make luminescent nanosheet paints that give out a widespread of emission colors. It is also possible for one single substrateto retain not only colors but also magnetic features, catalyticfeatures, etc. in various ways.

1. A luminescent nanosheet having perovskite octahedral crystalscombined together in a planar configuration, characterized in that saidoctahedral crystals each have a multistacked crystal sheet structure,wherein said octahedral crystals are multistacked over at least 3 highin a direction vertical to a sheet plane, and an element providing aluminescence center is solid-solubilized between the multistackedoctahedral crystals.
 2. The luminescent nanosheet according to claim 1,characterized in that said perovskite octahedral crystals are eachcomprised of a tantalum oxide or a niobium oxide, and said luminescencecenter element is a rare earth element (ion).
 3. A fluorescentilluminator comprising a fluorescent substance that receives excitationenergy from an excitation source to emit out visible light having agiven wavelength, characterized in that said fluorescent substance is aluminescent nanosheet as recited in claim
 1. 4. A solar cell that uses aphotoelectric device capable of generating electricity upon receipt oflight and has on a light-receiving surface side of said photoelectricdevice a photofilter that is excited by solar light to emit out lighthaving a wavelength different from that of solar light, characterized inthat said photofilter comprises a luminescent nanosheet as recited inclaim
 1. 5. A color display comprising a luminescent substance thatreceives distinct excitation energies from distinct excitation sourcesto emit out light in distinct colors, characterized in that saidluminescent substance comprises a luminescent nanosheet as recited inclaim 1, or a combination of it with other luminescent nanosheet orother luminescent substance.
 6. A nanosheet paint having a nanosheetdispersed in a disperse medium, characterized in that an X value in thefollowing equation 1 found from a vapor pressure of the disperse mediumand a concentration of the nanosheet is in a range of less than 4.9×10⁶to greater than 3.8×10³:X=C×V ^(4.01)   1 where C is the concentration (M) of the nanosheet, andV is a saturation vapor pressure (torr) of the solvent at 25° C. and 1atm.